Since the 1940's optical devices in the form of intraocular lenses (IOLs) have been utilized as replacements for diseased or damaged natural ocular lenses. In most cases, an intraocular lens is implanted within an eye at the time of surgically removing the diseased or damaged natural lens, such as for example, in the case of cataracts. For decades, the preferred material for fabricating such intraocular lenses was poly(methyl methacrylate) (PMMA), which is a rigid, glassy polymer.
Softer, more flexible IOLs have gained in popularity in recent years due to their ability to be compressed, folded, rolled or otherwise deformed. Such softer IOLs may be deformed prior to insertion thereof through an incision in the cornea of an eye. Following insertion of the IOL in an eye, the IOL returns to its original, pre-folded shape due to the memory characteristics of the soft material. Softer, more flexible IOLs as just described may be implanted into an eye through an incision that is less than 4.0 mm i.e., much smaller than the 5.5 to 8.0 mm incision necessary to implant more rigid IOLs such as those made from PMMA. A larger incision is necessary for more rigid IOLs because the lens must he inserted through an incision in the cornea slightly larger than the diameter of the inflexible IOL optic portion. Accordingly, more rigid IOLs have become less popular in the market since larger incisions have occasionally been found to be associated with an increased incidence of postoperative complications, such as induced astigmatism.
With recent advances in small-incision cataract surgery, increased emphasis has been placed on developing soft, foldable polymer materials suitable for use in artificial IOLs. In general, these materials fall into one of three categories: hydrogels, silicones and low glass transition temperature acrylics.
A further recent advance in IOL implantation is the use of IOL injectors to implant the IOL in the eye. Cf., US 2007/0060925 “Preloaded IOLS injector and Methods” to Pynson; US 2005/0222578 “IOL Injector” to Vaquero; and U.S. Pat. No. 7,988,701 “Preloaded IOL Injector” to Vaquero et al.; each of which are incorporated by reference herein in their entireties. Unfortunately injector implantation of an IOL generally proceeds more smoothly (i.e., with fewer surgical difficulties) the more rigid and thus generally the more handleable (manageable), the IOL.
Thus, for surgical purposes, a more rigid lens is suggested. Usually this means a less than fully hydrated polymer lens is injected. As is well known, post-implantation hydration of an IOL changes, sometimes unpredictably, the refractive index (RI) of the lens. This subjects the physician and the injectable IOL implantation to uncertainty as to the surgical outcome.
In general, high water content hydrogel materials have relatively low refractive, indices, making them less desirable than other materials with respect to minimal incision size. Low refractive index materials require a thicker IOL optic portion to achieve as given refractive power.
Silicone materials may have a higher refractive index than high-water content hydrogels, but tend to unfold too rapidly after being placed in the eye in a folded position. This can be a problem because a rapid unfolding of a folded lens can potentially damage the corneal endothelium and/or rupture the natural lens capsule and associated zonules.
Low glass transition temperature acrylic materials are desirable because they typically have a high refractive index and unfold more slowly and more controllably than silicone materials when inserted into e.g., the lens capsule. Unfortunately, low glass transition temperature acrylic materials, which contain little or no water initially, may absorb pockets of water, in vivo, causing light reflections or “glistenings.” Furthermore, it is difficult to achieve ideal folding and unfolding characteristics due to the temperature sensitivity of acrylic polymer memory.
US. Pat. No. 5,480,950 issued Jan. 2, 1996 discloses high refractive index hydrogel materials having a hydrated equilibrium water content (“EWC”) of at least 57% for use in the manufacture of IOLs. The high refractive index hydrogel materials are cross-linked polymers prepared from mixtures of N-vinylpyrrolidone, 4-vinylpyrimidine and a vinyl pyridine having equilibrium water contents up to 90% and refractive indexes of 1.560 to 1.594 in the dry state. The IOLs as described are not implanted in a hydrated state. Rather, the IOLs are implanted in a dry, folded and elongated state and hydrated in situ. The refractive indexes in the hydrated state as used in the eye are not provided. U.S. Patent Application Publication 2002/0049290 relates to high refractive index (RI) ophthalmic hydrogel materials.
U.S. Pat. No. 5,693,095 issued Dec. 2, 1997 discloses high refractive index, low water content IOL materials. The materials taught in this particular patent are acrylic materials having an elongation of at least 150%. IOLs manufactured from as material having such elongation characteristics will not crack, tear or split when folded. However, such low water content acrylic materials have been found to be less biocompatible than other materials when manufactured into and used as IOL devices.
In the past decade, hydrophobic polymers have been used in IOL manufacturing with some success. The ophthalmic community has accepted this type of polymer as having good physical properties and acceptable biocompatibility in ocular environments. However, current IOLs made from conventional hydrophobic polymers sometimes suffer from poor optical stability in ocular fluids (e.g. glistenings, optical artifacts) and low refractive indices. The formation of unwanted particles and deposits in the bulk of hydrophobic polymers is attributed to uncontrolled water sorption and subsequent phase separation. Conventional homopolymers currently used to produce copolymers with high RIs (>1.51) absorb varying amounts of water in a sporadic fashion, creating phase separation, haze, and glistenings.
Currently, there are no foldable, high RI IOL polymers that resist the formation of glistenings and deposits. Compositions known to resist formation of glistenings require hydration prior to implantation. This limits foldability, incision size, and preloading packaging, which quickly is becoming the method of choice for packaging IOLs. More importantly, there are no IOLs made with polymers with EWC having a value of in the range of about 3% to about 15% by weight. Not wishing to be bound by any theory, it is believed, however, that this family of polymers is more resistive to glistenings. Compositions, polymers, and methods to manufacture glistening-free IOLs with EWC of 5-15% are provided.
An advantage of the compositions and methods disclosed herein is a reduction or elimination in the uncertainty of surgical outcome in the context of a post-implantation hydratable or hydrating IOL polymer, particularly where implantation is accomplished using an IOL injector.